Implantable pulse generators have been used to provide electrical stimulation to organs, tissues, muscles, nerves, or other parts of a patient's body. One example of electrical stimulation is cardiac pacing. Cardiac pacing includes electrically stimulating the heart when heart's natural pacemaker or conduction system fails to provide synchronized atrial and ventricular contractions at appropriate rates and intervals for the patient's needs. When the patient's heart is beating too slowly, bradycardia pacing increases the rate at which the patient's heart contracts to provide relief from symptoms associated with bradycardia. Malignant tachyarrhythmia, for example, ventricular fibrillation (VF), is an uncoordinated contraction of the cardiac muscle of the ventricles in the heart, and is the most commonly identified arrhythmia in cardiac arrest patients. If this arrhythmia continues for more than a few seconds, it may result in cardiogenic shock and cessation of effective blood circulation. As a consequence, sudden cardiac death (SCD) may result in a matter of minutes. Cardiac pacing may also provide electrical stimulation intended to suppress or convert tachyarrhythmias. This may supply relief from symptoms, and prevent or terminate arrhythmias that could lead to sudden cardiac death or the need to be treated with high voltage defibrillation or cardioversion shocks.
Traditional implantable pulse generators include a housing that encloses a pulse generator and other electronics, and is implanted subcutaneously in the chest of the patient. The housing is connected to one or more implantable medical electrical leads. The electrical lead includes one or more electrodes on a distal portion of the lead that is implanted inside the patient, such as inside the patient's heart (e.g., such that at least one of the electrodes contacts the endocardium), within vasculature near the heart (e.g., within the coronary sinus), or attached to the outside surface of the heart (e.g., in the pericardium or epicardium).